By A. McMARTIN.
It is usually assumed that the early stages of a crop play some role in determining the ultimate yeild, but to what extent various factors which influence early development produce effects which are maintained till cutting time is not always quite obvious.
1 he following observations are of the nature of a summary of data collected over several years from experiments in which a study has been made of germination, tiller formation, and yield.
Arising out of these studies, the use of a fungicidal dip as a pre-planting treatment for cane setts has been recommended as being the means of producing the biggest improvement on a stand of cane under our conditions.
In the present paper some of the other factors which influence the development of a crop will receiv e consideration. For a start, some basic proper- ties of the propagation of cane from cuttings will be examined.
Regeneration in Plants.
A complete plant consists of three basic types of organ—roots, stems, and leaves—and the vegetative propagation of plants is made possible by the faculty possessed by many of producing missing organs when a portion of the plant is removed from its parent.
Thus when a leafy stem is removed and planted, roots will be produced; or when roots are planted, buds are formed which develop into leafy stems.
Portions of stems of some plants when planted with- out buds or roots can produce both. This property of producing, from new tissue, missing organs, to make a complete plant out of an incomplete one, is known as regeneration.
Polarity.
Regeneration does not proceed in a haphazard runner; the new organs are formed in a certain order.
If we consider the plant as having a main axis, or axes if more than one stem is produced, as in sugar-
cane, and the portion where stems and roots meet is being the centre, it is seen that the youngest regions, where growth is taking place, are those furthest from the centre; these regions we refer to as the distal regions, and those nearest the centre of the
plant the proximal regions.
When regeneration occurs now from a removed organ, buds develop at the distal end and roots at he proximal. This is so pronounced in some plants that stem cuttings must be planted with the distal and exposed and the proximal end in the soil; if
inverted, buds develop underground and roots on the exposed end. It will be seen, further, that whereas stem cuttings are planted in the same position as they were growing originally, root cuttings must be inverted, to bring the distal region uppermost. This phenomenon, known as polarity, is further shown if a stem cutting, for example, is cut in smaller pieces
—buds always develop at the distal end and roots at the proximal. The distal end of a cutting is thus the shoot pole and the proximal end the root pole.
With the sugarcane cutting, now, no true regenera- tion occurs, as the planted sett has buds and root primordia already formed, and waiting to develop.
In their order of development, however, polarity is exhibited.
If a stem be planted whole, shoot development begins at the top, or distal region, and proceeds down- wards, while root development begins at the bottom, or proximal end, and proceeds upwards. If, now, the cane is cut in two, each half behaves as the complete stem; similarly, cutting these halves further still preserves the orderly manner of development, in that the end of the cutting originally nearest the top of the cane behaves as a shoot pole, and the other end a root pole.
It should be noted that this does not mean that shoots or roots do not develop on the other regions of the cutting—polarity here refers to the order of development in time. Moreover, apart from this precision in the order of development relative to their position on the stem, the new shoots and roots develop in each sett in the same order of priority as that which would have been exhibited by that sett had it remained unsevered from the rest of the stem.
Thus, a cutting from the top of the stem not only develops shoots at the shoot pole first, but does so before roots develop from the root pole; while a cutting from the bottom of the stem develops roots from the root pole before shoots from the shoot pole.
The intermediate portions exhibit transitional stages between these two extremes.
An exception is found in the very young immature nodes at the apex of the stem, beneath the youngest green leaf sheaths. Here very vigorous development takes place both of shoots and roots together.
This phenomenen of the precise manner in which the development of buds into shoots occurs is related to internal physiological conditions of the stem, principally those which arise from the inhibiting effect of one developing bud on another. It is a
132 common observation that as long as the apical bud of a growing cane stem remains healthy or un- damaged, the lateral buds remain dormant, but that when the top is damaged, or checked severely in growth, development of the side buds begins, and from the top first. If these top developing buds are removed, the next ones lower down start into growth, and so on.
One explanation commonly accepted to explain this phenomenon is that bud inhibiting substances, of the nature of hormones, are produced by actively growing leaves which migrate down the stem and prevent the development of the buds in a proximal position. Thus the growing top prevents bud devel- opment lower down, but when removed, the next ones grow, and produce the inhibitors which prevent again the next lower ones from growing. It is also suggested that these bud inhibiting hormones are root stimulating ones, which accumulate at the basal end of the stem and incite the development of the root primordia into roots.
This inhibiting effect of one bud on another, or polar inhibition, can be overcome by cutting the stem into pieces, as in each piece the distal bud is free to develop without the inhibiting effect of the one above it. It is obvious also that by removal of the top of the cane the largest source of inhibitor is removed, as is seen by the fact that while in the standing crop the top completely prevents the de- velopment of lateral buds, in a cane sett the first developing bud does not prevent the development of the others, but merely retards them.
It was stated earlier that these hormones appear to migrate to the basal end of a cane stem; this refers to a growing stem, in a vertical position.
When the cane is planted it is laid horizontally, and in this position the hormones also migrate to the side of the sett which is now lowest. (This is a well- known feature of cuttings of other plants, where the root hormones accumulate at the lowest points of a horizontal cutting, and stimulate root development at these points.) If, now, the cane sett should be planted with one row of buds uppermost and one downwards, the latter are in the region of accumula- tion of the inhibiting hormones, and their develop- ment is more easily suppressed. Thus if a sett is planted with the top bud facing up, in developing first it inhibits the second bud, facing down, but allows the third bud, facing up, to develop next; the fourth bud, facing down, is nearly always completely inhibited. If the second bud faces up, it develops and inhibits the first and third. If, now, the sett is planted so that the buds face sideways, the normal order of development occurs, and the pronounced suppression of the buds facing downwards is re- moved. We may express the position thus: if the
buds are numbered 1 to 4 on a four-budded sett, their order of development is as follows :—
1. Top bud facing up—development order is 1-3-2-4.
2. Top bud facing down—development order is 2-4-1-3.
3. Buds sideways—development order is 1-2-3-4 The last bud is nearly always completely suppressed
Traumatic Response.
The division of the cane stem into pieces for propagation purposes involves the making of several cuts. In many cuttings, when a cut is made, re- generative changes occur at the cut end, leading to the formation of new tissue, which acts as a pro- tective barrier, and among other functions prevents the entry of micro-organisms into the cutting. This response to wounding, or traumatic response, is well seen in cuttings which produce a callus at the cut ends.
With sugarcane, however, no new tissue formation occurs; indeed, traumatic response appears to be absent, so that the cutting has no efficient barrier to prevent micro-organisms gaining entrance from the soil. One possible defence may be the fermentation which sets up at the cut end, resulting in conditions on the wound which are unfavourable for the growth of soil organisms. Whether this be the case or not, it does not appear to be a very efficient means, as cutting decay is easily caused by organisms pene- trating the wound. In fact, observation suggests that under Natal conditions one of the most common causes of failure of buds and roots to grow on a cutting can be traced back to the activity of micro- organisms destroying the cutting internally via the cut end.
Development of Tillers.
The shoot that emerges directly from the bud becomes the primary shoot, or mother shoot, of the group of canes which eventually arise from that bud, the others arising by branching from the base of the primary shoot. At the same time, roots are formed from the base of the shoots, resulting in these shoots eventually becoming independent of the sett and the sett-roots. The production of these shoot roots now are influenced to some extent by the manner of development of the shoots. As was stated earlier, the bud-inhibiting, but root-forming, hormones tend to accumulate on the basal regions, hence the buds which have developed from under the sett, and have curved round to emerge above ground, tend to pro- duce roots more vigorously than those shoots on the top of the sett.
The most uniform shoot root development comes from those setts in which the buds have been planted sideways.
133 Thus the orientation of the sett in relation to the soil can determine not only the order of development of the buds, but the vigour of shoot-root production.
The number of tillers now which arise from primary shoots can be influenced by the development of these primary shoots. Each tiller arises from a small bud at the base of the primary shoot, hence the greater number of buds underground on the latter the more tillers can be produced. The length of underground primary shoot, and hence number of buds, in turn can be influenced by the manner in which it emerges from the sett. Thus a shoot which grows straight up from the sett has not so many underground buds as one which has to grow for some distance under- ground before turning upwards, such as a shoot which has to grow beneath a leaf sheath, when the trash is left on the sett, before turning upwards.
Hence, leaving the trash on a sett produces primary shoots with long regions underground from which e\ entually a thicker crop of tillers emerge than when the trash is removed, permitting the primary shoot to emerge directly from the bud. The removal of trash, however, permits very much quicker emer- gence, and gives a much earlier crop of primary shoots This aspect will be referred to again.
Germination under Field Conditions.
The developmental history of a cane sett into a young established plant as has been described applies to those growing under conditions in which all buds have a good and equal chance of developing. Under field conditions, however, this is not attained, and moitality of buds is higher. An investigation some years ago into the percentage of buds which grew
showed that an average germination was only 26 per cent , reaching a maximum of over 70 per cent, under very favourable conditions. It was further found that germination of the buds on setts cut from the lower portions of a stem was 50 per cent, of that from top setts, and also that on a four-budded sett the germination from the two outside buds was only 50 per cent. of that of the inside buds. Thus, on a four- budded sett, the two inner ones have the greatest chance of survival, and of these the one that de-
clops first is in the position of being able to suppress the other one. The higher mortality of the outside buds appears to be due to its proximity to the fer- menting cut end, and possibly also to a certain amount of drying out.
The possibility of cut cane tissue when fermenting being toxic to buds was suggested by an experiment in which some cane stems were shredded and spread in a thin layer over the soil in pots in which setts
were planted ; in addition, another series was planted with the shredded cane mixed with soil, and a third had no shredded cane. The germination was as follows:—
Cuttings in soil alone 52percent.
Cuttings in soil plus shredded cane on
surface 8percent.
Cuttings in soil plus shredded cane mixed with the soil 2 per cent.
The toxic effect of cut cane tissue on germination now raises the question of the cutting of a cane stem into setts, as, obviously, the greater the number of setts cut from a stem the more fermenting surfaces are created; on the other hand, cutting counter- balances polar inhibition, and gives a larger number of buds a greater chance of developing, soil condi- tions apart. If a whole cane is planted—with, of course, the tops removed—the distal buds will be- come established, and if conditions are favourable bud development can proceed towards the proximal end; the latter buds will, of course, be slower, and if unfavourable conditions should be encountered may not germinate at all. At the other extreme, if a cane is cut into single-budded setts, all polar inhibition is removed, and all buds may grow with equal vigour; to obtain a good germination with single-budded setts, however, very good conditions are required.
This technique is regularly used in pot work, but has also been used successfully occasionally in the field where the best germinating conditions have been attained. For average conditions, however, it appears advisable to strike a compromise.
As was stated earlier, mortality of buds on the outside of a cutting may be high, so that only the inside ones should be counted as having a good chance of survival.
In a three-budded cutting this only leaves one, but in a four-budded sett there are two buds with a good chance of developing. For this reason, for con- ditions which, as they are frequently in Natal, are distinctly sub-optimal for germination, a four-budded cutting might be considered the average type for field planting. As was stated earlier, field counts showed an average germination of 25 per cent.—each four-budded cutting producing one shoot would yield this figure.
To investigate the effect on germination of setts planted in different positions, experiments have been carried out in which the setts have been dug up after germination was complete, and examined.
In one, recently completed, with N:Co.310, 1,200 four-budded setts were examined, and the following information was obtained : The total number of buds which grew, out of 4,800, was 3,730—about a 77 per cent. germination, which gave a very good stand of cane. Of these, setts planted with the buds sideways produced 1,267 primary shoots, with the top bud placed up 1,227 shoots, and with the second bud up 1,236 shoots. There was thus very little
134 difference between the different methods of placing the setts as far as total germination is concerned. It was obvious, however, by observation that the setts with the buds placed sideways had most roots on the young shoots. Taking the figures for the position of the bud on the sett, it was seen that out of 1,200 buds which were at the distal end of the sett 972 grew, while 1,099 second buds, 1,048 third buds and 611 fourth buds grew. Thus the highest germination was obtained with the two middle buds, and of these the second one produced more shoots. When poor germination is obtained, now, it is often found that the outside buds fail completely, and only one bud is produced, usually the one in the second position.
It has further been found that the effect of disin- fecting a cutting is to give a greater chance to the end buds to develop.
This effect is additional to the use of disinfectants for the control of pineapple disease; when this disease is present, it can penetrate a sett so com- pletely that the inside buds are as liable to decay as the outside ones.
Thus in one experiment with Co.331, which is extremely susceptible to pineapple disease, out of 400 setts only 85 grew when no disinfectant was used, and 180 of the dead setts showed positive signs of pineapple disease, while of disinfected setts 244 grew and only 14 showed the symptoms of that same disease. Of the untreated setts, the germination of buds was only 9 per cent., which consisted of 66 out- side buds and 73 inside buds, while the treated setts produced a germination of 24 per cent., consisting of 218 outside buds and 164 inside ones; thus here the outside buds germinated even better than the inside ones.
It is interesting to note the relationship between primary shoot production and tillering. Figures showing germination percentages merely show the number of buds that grow, and do not take into account the rate in which they have been pro- duced. The importance of this latter aspect of the subject was well shown in an experiment comparing the effect of different disinfectant treatments on the setts before planting. Here, untreated cane ger- minated to the extent of 29 per cent., while treatment with a disinfectant "A" increased it to 61 per cent,, and with disinfectant " B " to 54 per cent. "A" thus appeared better than " B , " but after two months the
" B " plots in the trial had an average of nearly five times more tillers than the "A" plots. This was seen to be due to the slower rate of germination of the
"A" treatment, although it ultimately reached a higher figure for germination percentage; the "A"
treatment took twenty days to complete germina- tion after the appearance of the first shoots, whereas the " B " treatment was finished in twelve days. If we divide the final germination figure by the number
of days to complete germination, giving a figure representing germination percentage per day, a figure which we might call the rate of emergence, the "A"
treatment had a rate of emergence of 3 per cent., the
" B " treatment of 5 per cent., and the untreated cane of 2 per cent.
If we divide the number of tillers by the number of primary shoots at two months, we see that "A"
treatment gave a figure of 0.5, while that of " B "
treatment was 2.4. It thus appears that figures showing germination percentage by themselves arc misleading—the important figure is that of the rate of emergence.
The final outcome of germination, of course, is the stand of shoots produced, which appears to be at its maximum at from two to three months, depending upon growing conditions—hence the value of showing the total population of shoots at that time, as is now done in our trials in which various fungicidal treat- ments are compared one with another.
It should be noted that strict comparison can only be made between setts treated similarly as far as trash removal is concerned; as was stated earlier, the exception to the rule of quickly-emerging shoots tillering better is when the trash is left on, as these eventually produce more tillers, although slower in emerging.
An interesting observation was made, while ex- amining the developing shoots from dug-up cuttings, on the relationship between the number of primary shoots developed per cutting and the number of tillers per primary shoot. One might imagine that if a cutting produced only one primary shoot, there would be more room and food material for the development of tillering than if, say, four primary shoots developed; examination, however, showed the following to be the case :—
Average number of tillers per primary shoot where one primary shoot developed, 2.5.
Average number of tillers per primary shoot where 2 primary shoots developed, 3.7.
Average number of tillers per primary shoot where 3 primary shoots developed, 3.6.
Average number of tillers per primary shoot where 4 primary shoots developed, 5.2.
Thus the larger the number of primary shoots a cutting produced, the greater the tillering.
The effect of disinfecting cane on this phenomenon was also of interest. Similar figures for cane treated with a fungicidal dip were as follows :—
Average number of tillers per primary shoot where 1 primary shoot developed was 4.7.
Average number of tillers per primary shoot where 2 primary shoots developed was 4.0.